1. Field of the Invention
The invention relates to computer data storage and, in particular, relates to a disk drive having a split VCM actuator that provides advantages in performance and cost savings.
2. Description of the Related Art
Disk drive storage devices are an important component in virtually all computer systems. In particular, disk drives provide computer systems with the ability to store and retrieve data in a non-volatile manner such that the data is maintained even if power is removed from the device. The popularity of these devices is based on their ability to quickly store and retrieve large quantities of digital information at low cost.
The typical disk drive comprises one or more pivotally mounted disks (also referred to as platters) having a magnetic recording layer disposed thereon and one or more magnetic transducer elements for affecting and sensing the magnetization states of the recording layer. Typically one transducer is associated with one magnetic layer of the disk. Thus, for example, a single disk having two recording layers has one transducer disposed adjacent each of the two recording layers, for a total of two transducers.
The recording layer comprises a large number of relatively small domains disposed thereon that can be independently magnetized according to a localized applied magnetic field and that can be maintained in the magnetized state when the external field is removed. The domains are grouped into concentric circular tracks each having a unique radius on the disk, and data is written to or read from each track by positioning the transducer over the disk at the corresponding radius while the disk is rotated at a fixed angular speed.
To position the transducer with respect to the disk, the typical disk drive further comprises a pivotally mounted actuator, typically using a pivot bearing. The transducer is typically mounted on one end of the actuator, and the other end of the actuator comprises a coil that forms part of a voice coil motor (VCM). The actuator is mechanically balanced with respect to the pivot bearing so as to facilitate rotation of the actuator, and thus the movement of the transducer, by a torque exerted by the VCM in a manner known in the art. To apply the torque to the actuator in a controlled manner, the disk drive further comprises a servo-controller for controlling the VCM. The VCM comprises a coil of conducting wire wound into a plurality of loops and a permanent magnet disposed adjacent the coil. The servo-controller initiates movement of the actuator arm by directing a control current to flow through the coil that generates a torque that causes rotation of the actuator about its pivot bearing. Because the direction of the torque is dictated by the direction of control current flow, the servo-controller is able to move the transducer to a different location by first directing the control current through the coil so as to angularly accelerate the actuator in a first direction and then reversing the control current so as to angularly decelerate the actuator.
The movement of the transducer in the foregoing manner is known as a seek operation, wherein the transducer is moved from a first track location to a second track location. The distance between the first and second track locations is known as a seek length, and is typically expressed as the number of tracks between the first and second track locations. The time required to complete the seek operation is known as a seek time, and the seek time is one of the parameters that contribute to the overall performance of the disk drive.
A traditional actuator that performs the aforementioned seek operation is typically configured such that all the transducers mounted thereon are substantially fixed relative to each other. As a result, the transducers mounted on the common actuator move in unison during seek operations. Such fixed configuration of the transducers and the common actuator has drawbacks that are well known in the art.
One drawback associated with the common actuator with multiple transducers relates to a relatively large moment of inertia resulting from such a configuration. As is understood in the art, moment of inertia of a rotating object is inversely proportional to its angular acceleration resulting from a given applied torque. Thus the common actuator having a relatively large moment of inertia accelerates at a lower rate, disadvantageously resulting in longer seek times. One way to compensate for the relatively large moment of inertia of the actuator, so as to achieve greater acceleration, is to increase the torque applied to the actuator. As is also understood in the art, such increase in applied torque disadvantageously requires greater power expenditure or use of higher torque generating (and higher cost) magnets.
Another drawback associated with the common actuator relates to the common motion of the transducers during seek operations. Because the transducers move in unison, seek operations result in “dead times” during which data is not transferred between the transducer(s) and the recording layers (read or write). Data transferred between the transducer(s) and the recording layers (read or write) is called a disk data transfer. The dead times disadvantageously leave gaps interspersed between active time segments of available data transfer time that affects the rate at which information can be transferred between the disk and a host computer (referred to as throughput). Host to disk data transfers are distinguished from internal transfers by calling them host data transfers.
To maintain a specified throughput capability of the disk drive, the dead time gaps can be compensated for in various manners. A general solution is to increase the rate at which data is transferred from the disk to the host computer during the “live” (non-dead) times. This objective can be achieved by increasing the bandwidth of a data channel through which the data to and from the disk is processed. Concurrently, the rate of data transfer between the transducer and the disk can be increased. One way to achieve increase in the data transfer rate between the transducer and the disk is to increase the rotational speed of the disk. As is understood in the art, increasing the rotational speed of the disk increases the rate at which the transducer interacts with the magnetic domains disposed on the tracks.
Spinning the disk faster, however, has drawbacks. For example, spinup time is longer for a faster spinning disk. Also, the track density of the disk, typically expressed as tracks per inch (TPI), needs to be decreased in order to accommodate effects associated with increase in rotational speed of the disk. These effects include aerodynamic vibrations between the transducer and the disk, bearing vibrations, and other disturbance producing forces. Thus in order to maintain a specified storage capacity of the disk drive, the number of disks needs to increase to compensate for the reduction in TPI. Such an increase in the number of disks, along with the increase in rotational speed, lead to additional heat generation and acoustic effects that need to be dealt with. Furthermore, material cost associated with additional disks is a substantial amount.
From the foregoing, it will be appreciated that various solutions are implemented to compensate for the dead time gaps in data transfer associated with the common actuator. Each of the various solutions described has advantages and drawbacks associated with it. Hence, as is well known in the art, a great deal of effort is made in the disk drive industry to reduce or eliminate the seek time.
One solution proposed for reducing or eliminating the dead time gaps in data transfer is to perform overlapping seek operations using two or more independent actuators. The dead time gaps may be eliminated in such a multiple actuator disk drive by, for example, performing a seek operation with a first transducer mounted on a first actuator while a second transducer mounted on a second actuator is performing a data transfer operation. Thus, seek operations performed by each of the actuators is hidden such that dead time gaps do not exist in the overall data transfer. Examples of disk drives that utilize multiple actuators and thus are adaptable for such hidden seek tasks are disclosed in U.S. Pat. No. 5,343,345 to Gilovich, U.S. Pat. No. 5,761,007 to Price et al (assigned to International Business Machines Corporation), and U.S. Pat. No. 5,901,010 to Glover et al (assigned to Cirrus Logic, Inc.).
Unfortunately the independent actuators exemplified in the disclosed patents suffer from drawbacks, including mechanical disturbance crosstalk between the two actuators. Specifically, each of the two actuators is mounted on its own pivot bearing assembly, typically in the form of a ball bearing. Each actuator and bearing generates a mechanical disturbance that is independent of the other actuator and its bearing. The two sources of vibration are known to combine in a manner that makes compensation of each transducer substantially difficult. Expected performance gains have not been realized because of these problems, and the dual actuator drives have not done well in the market as a result.
Thus from the foregoing drawbacks associated with the common actuator and problems encountered by proposed solutions, it will be appreciated that there is a continuing need for improving the manner in which transducers are moved. To this end, there is a need for an apparatus that allows seeks to be overlapped effectively so as to reduce or eliminate dead time gaps in the data transfer to and from the disk. There is a need for an actuator that performs such operations while effectively compensating for the debilitating vibrations.